1. Field of the Invention
The present invention relates to a highly integrated memory device, and more particularly to a ferroelectric capacitor memory device capable of improving an electrical interconnection between a lower electrode and an active region of a cell transistor.
2. Description of the Related Art
In general, a Pt film has been widely used as a lower electrode in highly integrated DRAM cells employing high dielectric materials as well as non-volatile memory device employing ferroelectric materials such as BST[Ba(Sr,Ti)O.sub.3 ].
FIG. 1 is a cross-sectional view of a conventional ferroelectric memory device having a Pt film as a lower electrode of a capacitor. As shown in FIG. 1, a capacitor in the conventional memory device is made up of a polysilicon plug 6, a diffusion preventing film 7 and a lower electrode 8 such as a Pt film. Since the Pt film, which is commonly used as the lower electrode 8, doesn't act as a barrier film preventing oxygen atoms from diffusing into its underlayer, the oxygen atoms are diffused into a diffusion preventing film 7 through the Pt film. In FIG. 1, unexplained reference numerals 1 denotes a semiconductor substrate, 2 field oxide film, 3 gate, 4 bit line, 5 interlayer insulating film, and 9 ferroelectric film.
A TiN/Ti film is widely used as the diffusion preventing film 7. The barrier metal films such as TiN and Ti films and the polysilicon film for plug vigorously react on oxygen atoms from the dielectric film so that the oxidation takes place at a relatively low temperature of about 500.degree. C. Accordingly, the electrical interconnection between the lower electrode and an active region of the transistor is broken down. With the increase of the deposition temperature of ferroelectric materials, this problem is getting more serious.
Particularly, in the case of ferroelectric materials, such as SrBi.sub.2 Ta.sub.2 O.sub.9 which is one of the prevailing materials for the ferroelectric capacitor, the temperature required in the deposition and crystallization is approximately 800.degree. C. Therefore, in order to fabricate the ferroelectric memory device on the COB (capacitor on bit line) structure, it is most important to electrically connect the Pt lower electrode to the active region of the MOSFET.